The present invention relates to a device for temporarily holding a lifting element on a metal surface of a form, during concrete pouring and setting phases of a prefabricated concrete member.
Handling of prefabricated reinforced concrete members requires the provision of lifting points of a sufficient number and correctly positioned in the members such that the reinforced concrete members remain stable when moved. The lifting points can be composed, depending on the configuration of the reinforced concrete parts, of either simple mild steel pins or by a variety of specific lifting elements, embedded in the concrete, associated with or without a reinforcing bar for anchoring.
The lifting systems in current use, use three technologies:
a sling screwed to a threaded socket;
a sling hooked into an anchor with an eye; and
a sling hooked into an anchor with a head.
In the case where the lifting element is composed of a threaded socket, it is proper during the molding of the concrete and the setting time thereof to ensure attachment of the socket to the form using suitable means, provided the forms can be made of wood, synthetic material or, more generally, metal. Known means permitting temporary holding of a threaded socket are composed for example of a simple bolt traversing the form and providing a grip for the socket by its threads. This method of attachment destroys the form and requires repairs for subsequent use, the repairs being long and expensive.
According to another possibility, the support for the socket does not pass through the form and is composed of a part that serves to hold the socket and a part that serves to hold the form temporarily. This socket support can be made of a single material, synthetic material or metal, for attachment to a wooden form, or from a material that has magnetic inclusions, a plurality of small magnets or a single magnet of larger diameter, for attachment to a metal form. The connection between the temporary socket support and the socket is produced by introducing a cylindrical part which can be threaded, or smooth, into the interior of the socket body. However, the systems with a smooth cylindrical part which lock by friction have the disadvantage of permitting the laitance to penetrate the threads of the socket when the concrete is being poured, especially when the latter is being vibrated. The laitance then hardens and prevents normal attachment of the sling by screwing. The poor engagement which then occurs often results in disengagement of the socket from the sling and consequently causes the concrete part being handled to fall, which poses serious risks to the personnel doing the handling. This lifting system is therefore in decreasing use.
Known means which permit temporary holding inside a form utilizing anchors with eyes and anchors with heads are as follows:
the hemispherical type made of polystyrene attached by gluing to the form and used only once; the hemispherical type made of steel, attached by screwing or welding, which damages the form and necessitates a prolonged and expensive repair for reuse; and
the hemispherical type made of flexible synthetic material requiring drilling of the form, and having the additional disadvantage of very rapid wear with a very limited number of applications due to the means for removal from the form which produce very high localized stresses in the material constituting the attachment.